1) Field of the Invention
The present invention relates to a developing unit that develops a latent image on a latent image carrier with developer carried on a surface of a developer carrier by magnetic force, and an image forming apparatus employing the developing unit such as a copying machine, a facsimile, and a printer.
2) Description of the Related Art
Conventionally, a developing unit of a two-component developing system where a latent image carried on a latent image carrier such as a photosensitive member is developed using two-component developer containing non-magnetic toner and magnetic carrier that includes magnetic particles has been known. Such a developing unit has a developer carrier constituted of a non-magnetic pipe or the like, which includes therein a magnetic force generator such as a magnet roller, and a developer containing unit that contains two-component developer. The developer carrier is driven by a driving unit such that a surface thereof is moved in a predetermined direction. The developer carrier attracts and draws two-component developer on its surface due to magnetic force generated by magnetic poles of the magnetic force generator on a region thereof facing the developer containing unit. The drawn two-component developer constitutes a magnetic brush by spiking magnetic carrier along lines of magnetic force and is conveyed to a developing region which is a region facing the latent image carrier according to surface movement of the developer carrier. In the developing region, toner is transferred from the magnetic carrier surface to a latent image on the latent image carrier so that the latent image is developed to a toner image. Prior to the development, the magnetic brush is restricted in layer thickness thereon by such a restricting member as a doctor blade arranged to face the surface of the developer carrier via a predetermined gap, so that stabilization of a conveyance amount of toner to the developing region and enforcement of frictional charging are performed.
Toner used in the two-component developer or the like is added on its surface with additives such as silica, titanium oxide in order to improve its ability to disperse. These additives are weak in mechanical stress and they are burred in the toner or released from the surface thereof according to stirring the toner inside the developing unit, which causes degradation of the toner easily. The toner degraded due to the burring or releasing of the additives is poor in frictional charging performance so that a charge amount does not rise easily even if stirring is started in the developing unit. Toner adheres to a non-image portion of the latent image carrier to cause so-called background dirt, or the toner stays and accumulates on a surface of the developer carrier for a long term without contributing to development, which results in remarkable lowering of an image density.
On the other hand, it is known that the magnetic carrier in the two-component developer is also subjected to mechanical stress so that a surface layer thereof is gradually cut, which result in deterioration of frictional charging performance of the magnetic carrier. The magnetic carrier whose frictional charging performance has been deteriorated cannot charge toner excellently.
The two-component developer or one-component developer that does not include magnetic carrier deteriorates in a time elapsing manner by applying mechanical stress on toner or magnetic carrier (the one-component developer and the two-component developer are hereinafter called “developer”, collectively). In recent years, as further fineness or higher image quality have been demanded, it is a significant problem to suppress time-elapsing deterioration of the developer.
With a view to the above problem, Japanese Patent Application Laid-open No. H11-161007 proposes a developing unit of a two-component development system where a distal end edge portion of a magnetic plate serving as a restricting member has been chamfered. The Japanese Patent Application Laid-open No. H11-161007 describes that stress acting on two-component developer at a time of a restricting process can be reduced by performing such chamfering.
Japanese Patent Application Laid-open No. H9-146374 proposes a developing unit of a two-component development system which is provided with a magnet roller for developer retaining. The magnet roller for developer retaining faces a developing sleeve via a predetermined gap in an upstream side of a doctor blade serving as a restricting member in a rotating direction of the developing sleeve, and a surface movement thereof is rotationally driven in the same direction as a surface of the developing sleeve in the facing region. The Japanese Patent Application Laid-open No. H9-146374 describes that the magnet roller for developer retaining with such a constitution can securely reduce the amount of the two-component developer colliding against the doctor blade to decrease stress acting on the developer.
As described later, the present inventors have found that a principal cause of degradation of two-component developer lies in friction occurring between “an immovable layer” and “a following rotation layer” of the two-component developer on a surface of a developer carrier. The term “immovable layer” means a layer that hardly moves to stay at a position thereof abutting against a restricting member on the surface of the developer carrier. The term “following rotation layer” means a layer which is one (the surface side of the developer carrier) lower than the “immovable layer” and rotates actively in a following manner according to surface movement of the developer carrier. Even in a developing unit using magnetic one-component developer (magnetic toner) that does not contain magnetic carrier or non-magnetic one-component developer (non-magnetic toner), a material similar to the “immovable layer” is easily formed on an upstream side of the restricting member. As a conventional art where staying of non-magnetic one-component developer on the upstream side of the restricting position is noticed in view of stabilization of an conveyance amount of developer to the developing region, a developing unit described in Japanese Patent Application Laid-open No. H5-35067 has been known. The developing unit is provided with a cylindrical developer conveying member that rotates while facing a developer carrier via a predetermined gap on an upstream side of a restricting member in a direction of surface movement, or a surface movement direction, of the developer carrier. In the Japanese Patent Application Laid-open No. H5-35067, a layer comprising non-magnetic one-component developer staying on the upstream side of the restricting position is referred to as “an immovable layer”. The Japanese Patent Application Laid-open No. H5-35067 describes that the immovable layer is prevented from being formed by providing the developer conveying member.
As mentioned above, the Japanese Patent Application Laid-open No. H11-161007 and Japanese Patent Application Laid-open No. H9-146374 describe that stress acting on two-component developer can be reduced by the chamfer or the magnet roller for developer retaining. As a result of studies by the present inventors, however, it is found that there is a problem that degradation progress cannot be suppressed sufficiently or degradation progress may be accelerated on the contrary even in these developing units.
FIG. 1 is an enlarged view of a partial constitution of a conventional developing unit of a two-component developing system. In a developing unit 20 shown in FIG. 1, a developing sleeve 22 constituted of a non-magnetic pipe serving as a developer carrier is rotationally driven by a driving unit (not shown) in a clockwise direction in FIG. 1. A magnet roller 23 having plural magnetic poles arranged in a circumferential direction thereof, which serves as a magnetic force generator, is fixedly arranged inside the developing sleeve 22 so as not to rotate according to rotation of the developing sleeve 22. A doctor blade 24 serving as a restricting member is disposed above the developing sleeve 22 in FIG. 1 such that a distal end face thereof faces a surface of the developing sleeve 22 via a predetermined gap.
A developer containing unit that contains two-component developer is provided in a region (not shown) inside the developing unit 20. The developing sleeve 22 rotates while is pumping two-component developer on a surface of the developing sleeve due to magnetic force generated by a pumping magnetic pole disposed in a region (not shown) of the magnet roller 23, so that two-component developer inside the developer containing unit is drawn up. The two-component developer drawn up clusters magnetic carrier along lines of magnetic force extending from the magnet roller 23 to form a magnetic brush. The magnetic brush is conveyed toward the doctor blade 24 according to rotation of the developing sleeve 22. At a position where developer is restricted by the doctor blade 24, a root side portion (a portion positioned on a surface side of the sleeve) of the magnetic brush formed to be thick on the surface of the developing sleeve 22 easily passes through the gap but a distal end portion thereof abuts on the doctor blade 24. If the distal end portion merely abuts on the doctor blade, the portion is replaced with the following root side portion of the magnetic brush little by little while being circulated in a convection manner on the upstream side of the doctor blade 24 in the sleeve rotating direction, so that it can gradually pass through the gap. However, lines of magnetic force from the magnet roller 23 reaches not only a root side region of the magnetic brush relatively near to the surface of the developing sleeve 22 but also a distal end side region thereof relatively far from the surface of the developing sleeve 22. Accordingly, the distal end side portion of the magnetic brush which has abutted on the doctor blade 24 is continuously constrained thereat due to magnetic force to stay without causing convection and form an immovable layer Ly1. A following rotation layer Ly2 due to the root side portion of the magnetic brush which easily passes through the gap while rotating according to the developing sleeve 22 is formed below the immovable layer Ly1.
The present inventors have photographed and observed behavior of a magnetic brush around the doctor blade 24 by a high speed camera with a high magnification as regard an experimental machine of the developing unit 20 with such a constitution and have found the following phenomenon. That is, in a boundary of the immovable layer Ly1 staying on an upstream side of the doctor blade 4 without almost movement and the following rotation layer Ly2 actively moving according to the developing sleeve 22, both the layers are violently rubbed to each other. The present inventors have supposed that regarding mechanical stress on the two-component developer on the developing sleeve 22, mechanical stress acting when the developer passes through the gap between the doctor blade 24 and the developing sleeve 22 is larger than that acting due to friction between both the layers.
Therefore, the present inventors have researched the influence to degradation of the two-component developer due to friction between both the layers. Specifically, a cause of formation of the immovable layer Ly1 lies in that a distal end portion of a magnetic brush whose following rotation is blocked by the doctor blade 24 continues to be constrained thereat due to magnetic force from magnet roller 22. When magnetic force or a shape of a restricting magnetic pole Px positioned so as to face the doctor blade 24 within the magnet roller 22 or an upstream adjacent magnetic pole Py positioned at an upstream side of the restricting magnetic pole is adjusted, magnitude or shape of a magnetic field affecting on the distal end portion of the magnetic brush can be change. The present inventors have made a research on the degree of degradation progress of the two-component developer while changing the size or shape of the immovable layer Ly1 variously in this manner. It is that as the length Lx of the immovable layer in the rotational direction of the sleeve (in the direction of the following rotation) becomes longer, degradation of the two-component developer is accelerated. It has also been found that, even if the amount of the two-component developer forming the immovable layer Ly1 is relatively much, when the length Lx of the immovable layer is relatively short, i.e., the immovable layer Ly1 is formed such that its size is long in a lengthwise direction thereof, degradation of the two-component developer does not progress so much. On the other hand, it has been found that, even if the amount of the two-component developer forming the immovable layer Ly1 is relatively less, when the length Lx of the immovable layer is relatively long, i.e., the immovable layer Ly1 is formed such that its size is long in a widthwise direction, degradation of the two-component developer progresses rapidly. It is considered such a fact is because the magnetic force of the magnet roller 23 rather than the weight of the immovable layer Ly2 acts as a factor which increases frictional force per unit area between both the layers, and as the length Lx of the immovable layer becomes longer, a friction time between the both layers becomes long.
The developing unit described in the Japanese Patent Application Laid-open No. H11-161007 corresponds to a structure where the distal end of the blade has been chamfered at a position indicated with a one-dot chain line in FIG. 1 in the constitution shown in FIG. 1. The chamfering allows the magnetic brush to pass through the gap between the doctor blade 24 and the developing sleeve 22 smoothly. However, since almost all of the mechanical stress to the magnetic brush occurs due to friction between the immovable layer Ly1 and the following rotating layer Ly2, it is considered that, even if slight stress occurring when the magnetic brush passes through the gap is only reduced, degradation progress of the two-component developer cannot be suppressed sufficiently.
The developing unit described in the Japanese Patent Application Laid-open No. H9-146374 corresponds to a structure where a magnet roller for developer retaining has been arranged at a position indicated with a dotted circle in FIG. 1 in the constitution shown in FIG. 1. The magnet roller for developer retaining is rotationally driven in a counterclockwise direction such that a surface thereof facing the developing sleeve 22 is moved in the same direction as the sleeve. The magnetic brush on the surface of the developing sleeve 22 passes through the gap between the magnet roller for developer retaining and the developing sleeve 22 before it reaches a position where restriction is conducted by the doctor blade 24. In the region between the magnet roller for developer retaining and the doctor blade 24, excessive two-component developer is conveyed according to rotation of the magnet roller for developer retaining in a direction of arrow C in FIG. 1 to be returned back to the developer containing unit (not shown). Since the excessive two-component developer is returned in this manner, a possibility is made high that an immovable layer is hardly formed. However, the magnetic brush is eventually forced into the gap formed in a tapered manner by surface movements of both the magnet roller for developer retaining and the developer sleeve 22 approaching to each other in the region therebetween. It is thought that, when the magnetic brush is forced into the gap, remarkably large stress is imparted on the two-component developer. The excessive two-component developer which has passed through the tapered gap is caused to contact with a side face of the doctor blade 24 in a rubbing manner by the following rotation according to rotation of the magnet roller for developer retaining in a returning course in the direction of arrow C. As a result, there arises a problem that degradation progress of the two-component developer cannot be suppressed sufficiently, or it may be accelerated on the contrary.
On the other hand, the developing unit described in the Japanese Patent Application Laid-open No. H5-35067 employs a non-magnetic one-component development system. When the developer conveying member of the developing unit is adopted in the constitution of the two-component developing system shown in FIG. 1, a cylindrical member is eventually arranged at a position indicated with a dotted circle in FIG. 1 via a predetermined gap with the developing sleeve 22. The cylindrical member is rotationally driven in a clockwise direction in FIG. 1 such that its surface facing the developing sleeve 22 is moved in a direction reversed to the rotational direction of the sleeve. Such a cylindrical member is provided so that, when the developer is non-magnetic one-component developer, staying of the developer in the vicinity of the doctor blade 24 can be solved. However, the two-component developer or the magnetic one-component developer has a high possibility that it forms an immovable layer between the cylindrical member and the doctor blade 24, because excessive developer occurring slightly between the both is continued to be restrained thereat due to magnetic force of the magnet roller 22. There is a possibility that excessive developer that has not passed through the gap between the cylindrical member and the developing sleeve 22 is deposited to form an immovable layer even on the upstream side of the cylindrical member in the rotational direction of the sleeve. Even if the excessive developer is not formed in the immovable layer owing to circulating convection according to rotation of the cylindrical member, it is rubbed against the cylindrical member or the following rotation layer violently. As a result, there is found a problem that degradation progress of the developer cannot be suppressed sufficiently or it is accelerated on the contrary.